I have Tamiya double gearbox problem (pololu.com/product/114/specs)
I am using one of these gearboxes on my robot project. The gear ratio is 1:114.7. The gearbox is brand new and after ~20mins of operation, one of the motors became much slower. Both motors have 0.9 Ohm resistance, both work, but it became much slower. I tried with another SN754410NE driver, the result was the same. After i changed the motor with a spare one from another double gearbox kit all became normal again. Is that a brushed motor? If it is, is it possible that the brushes were destroyed after 20mins of operation? What could be the problem?

Those are brushed DC motors, but without knowing more, it is difficult to determine what might have happened. How much slower is the motor than it used to be? Could you tell me more about how you were using the motor? Is it possible that something could have happened to just the one motor in your setup? Was the motor ever stalled when you were using it? What kind of load did you have on it? What were you using to supply power?